In capturing physiological signals, obtaining a clear signal often poses a problem, as the signal is generally weak and drowned by background noise.
This difficulty increases when the signal is acquired in an MRI environment and when the signal involved is an electrocardiograph (ECG).
Not only is the electrocardiograph the preferred indicator of a patient's health, but it also serves as a sequencing signal, particularly for an MRI imager; for example, it may function as a signal for launching an acquisition sequence (better known as “triggering”) and/or a signal for launching an acquisition window (better known as “gating”).
In practice, the signal can be used in different ways, such as visualization, shape analysis, counting, or the like, in order to determine morphology, frequency, or other characteristics.
An electrocardiograph signal is a repetitive signal with each sequence consisting of a juxtaposition of several waves (P, QRS, T, ST).
Filtering signal noise is, of course, important in terms of monitoring. In actuality, in order to follow a patient's progress in real time, a doctor must have access to intelligible data; a noisy ECG is useless.
The detection of peaks in the QRS complex is of capital importance: first, in order to determine cardiac frequency, and second, to synchronize the capturing of images from an MRI imager with the ECG (“triggering”/“gating”). This cardiac synchronization allows each section to be excited at exactly the same moment in the cardiac cycle, and thus it provides an image of the section that is free of motion-related phenomena.
Currently the electrocardiograph signal-filtering phase is generally accomplished using analog circuits or dedicated chips with fixed characteristics and limited performances, which are not successful in effectively extricating the ECG signal from the artifacts and parasites arising from the electromagnetic turbulence that prevails, in particular, during MRI examinations.
These electromagnetic disturbances originate essentially from peaks and local changes in the principal magnetic field Bo from the action of gradients beginning during the MRI experience and introducing a noise signal in the measurement loop of the ECG signal, which may be equivalent in intensity or even louder than the ECG signal.
Note that Bo is oriented along the patient's longitudinal axis in the case of a tunnel MRI apparatus, and perpendicular to the patient's coronal plane in the case of an “open” type MRI apparatus.
Considering that the orientation of the principal field Bo corresponds to the axis Z of an orthogonal spatial index (X, Y, Z), the linear gradients at X, Y and Z can be represented as follows:             grad      .      X        =                  ⅆ        Bz                    ⅆ        x              ,            grad      .      Y        =                                        ⅆ            Bz                                ⅆ            y                          ⁢        and        ⁢                                   ⁢                  grad          .          Z                    =                        ⅆ          Bz                          ⅆ          z                    
In an attempt to suppress the noise signal induced, several solutions have been proposed.
For instance, it has been proposed to submit the ECG signal to a filter that is sensitive to the increasing voltage speed of the regulated signal at a value slightly higher than the typical maximum value of the dV/dt of the ECG.
However, this process does not suppress the noise induced by the application and suppression sequences of the different gradients of elevated frequency.
It has also been proposed to derive the acquired ECG signal into two secondary signals, to delay one of the latter signals for a duration corresponding to a multiple of the period of the ECG signal, to extract the noise component from it, and then perform a subtraction between the non-delayed signal and the noise component of a preceding period.
However, this method only leads to noteworthy improvement in the ECG signal if the induced noise, and thus the electromagnetic conditions, are essentially identical over several periods of the ECG signal and if the latter is relatively regular. In practice, this is not often true and a significantly distorted ECG signal may result.
The goal of the present invention is to overcome the difficulties enumerated above.